The helium mass spectrometric fine-leak test is one of the most commonly used methods for detecting the sealability of a sealed electronic component, and may be based on a helium pressurizing method or a helium prefilling method.
So far, in China National Standard, China National Military Standard, IEEE and American Military Standard for helium mass spectrometric fine-leak test, including MIL-STD-750F/750-1 and draft amendments MIL-STD-883J published in USA in recent years, the basic criterion employed for a sealability test method is often an equivalent air standard leak rate L, which may be adjusted for different cavity volumes. For example, in the second draft amendment MIL-STD-883J published in September 2012, the sealability test requirements are divided into nonspace-level requirements and space-level requirements from a small cavity volume to a large cavity volume, where the space-level basic criterion L is from 1×10−4 Pa·cm3/s to 1×10−3 Pa·cm3/s, and the minimum helium gas exchange time constant, i.e., a rigour grade τHemin, corresponding to the basic criterion L and the criterion R1 for measured leak rate is from 43 days to 8600 days, which differs by about 200 times; under the same environmental condition, the reliable storage life with an internal water vapor content no larger than 5000 ppm also differs by about 200 times; thus, τHemin and the reliable storage life are rather unbalanced. More notably, the maximum test-waiting time in the fixed scheme of such standard helium pressurizing methods is often qualitatively determined as “1 hour or 0.5 hour” and “1 hour”; when such standards are applied to a helium prefilling method, the maximum test-waiting time is determined as “immediately”, “0.5 hour” or “1 hour”. For a small component under test with a cavity volume less than 0.002 cm3 or less than 0.006 cm3, the corresponding test-waiting time is 0.5 hour or 1 hour, and a component under test with a large leak will be undetected. For compoments under test with a common cavity volume range, it is difficult or unable to lower the leak rate of surficially absorbed helium of the components to the low background leak rate level required for high-rigour grade sealability test during the maximum test-waiting time of 0.5 hour or 1 hour, and during batch tests, it is difficult to control a further background leak rate, i.e., leak detector background, thus limiting the number of components that can be detected in the same batch; thus, the operability when the criterion is made stricter in the above standards and the latest draft amendment thereof becomes problematic. Therefore, for the purpose of stricter criterion for measured leak rate with respect to sealability, classifying the sealability, extending and balancing the reliable storage life of a sealed electronic component, it becomes a key point to improve the basic criterion for helium mass spectrometric fine-leak test, especially to improve the method for determining the maximum test-waiting time and effectively lengthen the maximum test-waiting time.
In the prior art, it was put forward in 2009 to take τHe as a rigour grade, but the more precise rigour grade τHemin has not been introduced; approximate formula for calculating the maximum test-waiting time based on a pressurizing method and the maximum test-waiting time based on a filling method have been put forward, but the method for determining the maximum test-waiting time was not explicit. And an apparent difference exists between the connotation as well as the expression of the approximate formula of the maximum test-waiting time and those of the present accurate formula; when the maximum test-waiting time exceeds ( 1/10) τHemin, the difference becomes greater, and an undue limitation is laid on the cavity volume range applicable to the formula of the maximum test-waiting time. In some standards, a storage method is employed, where the atmospheric pressure in storage environment is P0, but the partial pressure of the helium gas is specified as 0.1P0; as a result, the judging result is changed in many cases.
Based on the above description, during the helium mass spectrometric fine-leak test of the sealability of a sealed electronic component, it is necessary to accurately determine the maximum test-waiting time, quantitatively determine the maximum test-waiting time and effectively lengthen the total time for storage and test waiting; moreover, it is necessary to improve the basic criterion for fine-leak test and the method for determining the criterion for measured leak rate.